1. Field of the Invention
The present invention relates to an apparatus and method for inerting a wave soldering installation having a solder bath and a conveying system for producing one or more solder waves, such as is used, in particular, for soldering electric printed circuit boards.
2. Description of Related Art
Wave soldering installations for soldering printed circuit boards are known, for example from U.S. Pat. No. 5,121,874. In the case of the system described there, the inerting of the atmosphere over the solder bath is achieved by an elongated, closed covering, within which the printed circuit boards to be soldered are transported and in which an inert atmosphere, generally nitrogen, is maintained. In the vicinity of the solder waves which are produced by the wave soldering installation during operation, there are porous pipes which run parallel to the waves and through which inert gas emerges, so that a particularly low-oxygen atmosphere can be achieved, in particular underneath the printed circuit board to be soldered.
In addition, U.S. Pat. Nos. 5,411,200 and 5,409,159 disclose the practice of surrounding distribution pipes for inert gas above the solder bath of a wave soldering installation with cage-like housings, which allow the inert gas to emerge through numerous outlet openings. In those documents, a description is also given of the possibility of operating such a wave soldering installation with only a short covering hood or even without a covering hood, by producing a very low-oxygen atmosphere underneath the printed circuit boards by means of skillful distribution of the inert gas above the solder bath during the passage of printed circuit boards to be soldered, and even in the state when no printed circuit board is passing through, by producing a protective layer of inert gas over the solder bath and the solder waves.
In addition, U.S. Pat. No. 5,361,969 also discloses the practice of designing an apparatus for inerting a wave soldering installation as an immersion box which is closed at the sides and in which pipes for distributing inert gas are mounted.
However, in the case of a wave soldering installation, three different operating states may occur, specifically standstill of the installation (that is to say pumps switched off and, accordingly, solder waves not present), installation operation without a printed circuit board passing through and installation operation with a printed circuit board passing through. For all three states, it is necessary to ensure that little oxygen gets to the surface of the solder bath and the solder waves. In addition, when a printed circuit board is passing through, it must be ensured that a particularly inert atmosphere with only very low proportions of oxygen of less than 10 ppm can be maintained underneath this printed circuit board, where the soldering process is intended to take place, since otherwise the quality of the soldered joints becomes poorer. In addition, it is intended that such an installation should be operated for as long as possible without maintenance and should be as easy as possible to maintain. It is precisely on this point that the known installations have weaknesses, since, for example, splashes of solder, which are always produced during the operation of a wave soldering installation, can strike the distributor pipes for nitrogen and block these up over the course of time. This is critical in particular when the distributor pipes used are porous pipes with very fine pores.